1. Field of the Invention
The present invention concerns a method of cooling a current feed between a terminal at ambient temperature and an electrical equipment operating at very low temperature and at variable current. This method exchanges heat between the current feed and an auxiliary cooling fluid. This auxiliary fluid is injected at ambient temperature at a point between the terminal at ambient temperature and the electrical equipment. The fluid outlet is at the terminal at ambient temperature. The invention also includes a device for implementing this method.
The invention applies in particular to cooling superconductive equipment, especially superconductive machines, operating at different times with no or only a low current, with a nominal current and with a current greater than the nominal current.
Current feeds of this kind connect an external circuit operating in an ambient temperature environment to the very low temperature electrical equipment, immersed in liquid helium at 4.2K or in liquid nitrogen at 77K, for example. They are subject to losses as a result of the Joule effect and by thermal conduction; these losses introduce an additional liquefaction-refrigeration overhead as compared with that for cooling the very low temperature equipment.
It is known that the liquefaction-refrigeration requirements balance (cryogenic balance) is improved when the current feeds are cooled by exchange of heat with an fluid injected in contact with them at low temperature, evacuated at ambient temperature and then cooled by the cryogenic machine cooling the equipment. The cryogenic fluid is usually evaporated by the current feed itself, which is then said to be "self-cooled". Under these conditions, it is known that the heat losses for a given current are minimal when the ratio ##EQU1## (where L and S designate the length and the transverse cross-section of the conductor) has an optimal value r.sub.opt depending only on the nature of the conductive material. In the case of a current feed whose transverse cross-section S(1) is not constant over the entire length of the conductor an analogous law is obtained: ##EQU2##
The definition of the current feed showing minimal losses therefore depends on the current I that it carries. In particular, a current feed optimized for a given current I.sub.l has non-negligible losses under no load conditions (approximately 50% of the full load losses) and may even be destroyed in the event of an overload for a current varying from 1.05 I.sub.l to 3 I.sub.l depending on its structure and the conductive material used.
It is therefore advantageous to modify the current feed according to the current I that it must carry in such a way as to maintain the above ratio r as close as possible to the value r.sub.opt.
A first proposal is mechanically to unplug the current feed when the electrical equipment is shut down. This makes it possible to reduce considerably the losses under no load conditions, but introduces problems of reliability and of contact resistance, and in the event of automated operation requires a complex control system and the provision of adequate room to maneuver. The current feed is then optimized only for a single value of current, unless it is subdivided into a plurality of individually unpluggable elements that are thermally insulated from each other, resulting in a very complex installation.
Another proposal is that the electrical contacts should be transferable between an over-dimensioned conductor at a temperature near ambient temperature and the current feed proper (a so-called "sliding" contact) so as to vary the useful length l such that its value remains close to the product r.sub.opt.s/I. This solution introduces problems of reliability, of electrical resistance and of effective sealing. It requires a large amount of room to maneuver above the cryostat and flexible or articulated conductors, and is therefore hardly feasible for use in a confined space.
Finally, the document U.S. Pat. No. 4 209 658 proposes a method of optimizing a current feed in which the point of the feed maintained at ambient temperature is moved by sliding in a bore internal to the feed a piston delimiting the extreme point that can be reached by cooling water introduced into the bore, so as to vary the length of the current feed below ambient temperature in inverse proportion to the current. A method of this kind has the same drawbacks as mentioned above except that concerning the contact resistance.
An object of the present invention is to optimize the current feed for a plurality of current values, for example a null or low value (operation of the very low temperature electrical equipment with no load), a value representing nominal operation and a value representing a temporary overload of the electrical equipment (starting).